In sprinklers, which constitute the main field of use for glass bulbs, such bulbs act as the thermally active release member to keep a valve closed. The elongate bulb is generally secured at its ends between two abutments of the sprinkler and such abutments apply a force or mechanical load endways or axially of the elongate bulb. In the case of a fire, the glass bulb shatters and frees the valve to open and to release the fire extinguishing medium, which is usually water.
Such a glass bulb typically comprises a hollow and generally cylindrical or barrel shaped enclose or shaft, the length of which may vary widely. The bulb is often provided with an annular offset or shoulder in the wall at one end of the shaft so as to form the actual thermally active part together with the expansible breaking fluid or liquid confined within the glass enclosure. At the ends, which engage sprinkler abutments, the shaft is bounded by flat, conical or curved, substantially thermally inactive ends. One of the ends is normally referred to as the tip end, which is thin and tapered to a rounded point. The expansible breaking fluid is introduced into the bulb through the tip end during manufacturing, and thereafter the tip end is closed.
The glass bulb must be able to take a specific permanent load which is dependent upon the nature of the valve construction or release mechanism in the sprinkler as to insure that the sprinkler remains reliably closed over several decades and is always kept in a state of readiness.
Previously known glass bulbs which satisfy the appropriate standards imposed by insurance or governmental agencies, generally have a diameter between 8 and 12 mm, a wall thickness of 1 to 1.5 mm, and an overall length of 20 to 30 mm. Such relatively thick glass bulbs do not respond quickly to the application of heat from a fire, but have rather long release times, i.e., the time lapse from the first occurrence of critical temperature to be sensed to the shattering of the bulb and release of the valve. Such long release times are a result of the unfavorable ratio of the heat-absorbing surface of the bulb to the volume within the bulb to be heated. A spherical glass bulb in this respect indicates the least favorable shape.
The commercial market demands regarding glass bulbs for sprinklers for automatic fire extinguisher systems and also for other thermal release means, are for much shorter release times, which may be up to almost ten times shorter. Such shorter release times must be achieved without sacrificing durability of the glass drum to axial loading in the sprinkler.
One prior proposal to meet these requirements consisted of reducing the volume of breaking liquid in the glass bulb by placing a solid displacement member in the bulb without modifying the dimensions of the glass body, and therefore without modifying the strength characteristics. See U.K. Patent No. 2,120,934, published Dec. 14, 1983. Attempts have also been made to reduce the release times by reducing the overall diameter of glass drum so as to bring about a more favorable ratio of the surface area to the volume of the bulb, and consequently of the volume of the breaking liquid in the bulb. However, these attempts have lead to an unacceptable reduction in strength.